Combat vehicles typically have vision blocks, periscopes or other optical instruments with which a soldier in the vehicle can look out of the vehicle during combat. One danger of using these instruments is an enemy laser beam directed at the instrument to blind the soldier using it. The laser beam is typically "agile" in that it can vary greatly in intensity, pulse length, and wavelength, and current viewer protection technology can not protect the viewer over the entire range of agile laser wavelengths.
My invention uses a nonlinear absorbing or scattering material, or NLASM, to provide protection against agile lasers. NLSAMs absorb or scatter more completely at higher light levels than at lower light levels and include the following mechanisms: suspension cell limiters, two-photon absorbers, gas plasma breakdown cells, and non-linear index changing systems. For convenience there follows a brief discussion of each type of NLASM.
Suspension cell limiters. Suspension cell limiters are liquid filled cells having solid particles suspended in the liquid, the particles selectively absorbing light whose intensity exceeds some threshold level. The suspension cell limiter's liquid is usually an alcohol and the suspended particles are typically carbon, but other materials can be used. To use a suspension cell limiter, an incoming laser beam above the threshold intensity level is focussed to a point in the cell. Atoms of the particles in the suspension absorb energy from the laser beam. If enough energy is absorbed, electrons are freed from these atoms and form a collection of free ions called a plasma. The plasm then absorbs further energy from the laser beam.
A suspension cell limiter may be regarded as having four possible conditions, the condition at a given time depending on the intensity or fluence (light flow rate, expressed in Joules per square centimeter) upon the cell. The first condition exists when the fluence level is too low to cause plasma formation within the cell. In this case the cell is a passive light transmitter where the suspended particles filter out a percentage of the light that depends the particles concentration. The second condition occurs within a so called pre-clamping range of fluence levels, this range being above the threshold for plasma formation but insufficient to cause a phenomenon referred to as "clamping." Within the pre-clamping range of fluences, the plasma absorbs a part of the light attempting to pass through the cell, the increase in fluence attenuation by the plasma being roughly directly proportional to the increase in incident fluence. The third condition of the suspension cell limiter occurs at a critical level bordering the upper end of the pre-clamping range. At this critical fluence level, any increase in fluence incident upon the cell is completely absorbed by the plasma. In other words the cell's throughput of fluence is at a local maximum the upper end of the pre-clamping range. At a still higher critical fluence level, there occurs the limiter's fourth condition, where the limiter's plasma can absorb or block no more laser light. All fluence above this latter critical level passes unattenuated through the cell. The fluence values at which attenuation begins, clamping begins, and clamping ends depend on a number of parameters, including cell depth, the concentration of the suspended material, the type of suspended material, the liquid in the cell and the wavelength of the light. As an example, a typical suspension cell limiter has carbon particles suspended in alcohol at a concentration that filters out 10% of ambient light. Such a limiter typically begins fluence attenuation at fluences between 0.1 and 1 Joules per square centimeter. Clamping for such a suspension cell limiter occurs typically between 1 and 100 Joules per square centimeter, and clamping typically ends at 1500 Joules per square centimeter.
Two-photon absorbers. Normal linear absorption of light by a material, such as a colored dye, occurs when an incoming photon has an energy equal to that required to excite an electron in the material from a ground state to an excited state. This absorption of light is considered linear since a constant percentage of incident photons is absorbed regardless of the number of incident photons, at least until bleaching, which is not discussed here. Some materials exhibit nonlinear absorption if the necessary concentration of incident photons occurs. The atomic electronic structure for these materials may be such that the energy needed to transit one of its electrons to the first excited state may be close to, but not exactly equal to, the energy of a single incident photon, whereby the percentage of incident photons absorbed is low. However, this atomic structure may be such that the energy needed for transition of the electron from the first excited state to a second excited state may correspond well to the energy of the incident photon. Therefore, any electrons that absorb a photon and move from the ground state to the first excited state will surely absorb a second photon and move to the second excited state, if second photons are available. Material having such electrons absorbs nonlinearly since it absorbs a greater percentage of incoming photons when these photons are more concentrated. The differential absorption for more conentrated photons occurs because incoming photons are available for electrons that have absorbed a first photon and have not lost the energy thereof.
Gas plasma breakdown cell. In a gas plasma cell, a portion of high energy light incident thereon is absorbed by gaseous molecules in the cell. Electrons from the molocules form a plasma which absorbs and scatters the remaining incident light. At low incident light levels, there is not enough energy to induce a plasma and the light simply passes through the cell.
Non-linear index changing system. A non-linear index material has a first index of refraction for low energy light and a second index of refraction for high energy light. Such a material could be formed into small spheres and put into a cell containing fluid whose index of refraction matches the first index of refraction for the material. Since the spheres are index matched with the fluid, they will not be visible under normal viewing conditions. But when high energy light impinges on the spheres, their index of refraction changes and they become light scattering centers.